This is a continuation of a study which has been directed toward a thorough understanding of the biology of two organisms found in natural populations of related species of South and Central American Drosophila. The first of these is a spirochete-like or spiroplasma-like (mycoplasma) organism found to occur in the hemolymph of these flies. Its presence in the Drosophila female is responsible for the appearance of the sex ratio (SR) trait and results in the total elimination of the male sex from her progeny. The second is a mycoplasma-like organism found in gonadal tissue and which is associated with the occurrence of male sterility in F1 hybrids derived from crosses between strains belonging to two different semispecies. A number of aspects of the biology of these organisms are being studied: 1) gross and fine-structure morphology of the organism, 2) in vitro culture of the spirochete-like organism, 3) immunological studies of the possible relationship between these spiral-shaped organisms and the similarly-structured organisms described as causing corn stunt and citrus stubborn diseases, 4) electron microscopic studies of thin sections of larval and adult testes and ovaries and embryonic tissues (pole cells and the gonadal tissues to which they give rise) in which the mycoplasma-like organism is found, thus to elucidate the pattern of maternal transmission of these organisms, 5) in vitro culture of the mycoplasma-like agent by establishing primary cell cultures from infected flies, and 6) the possible relationship between the spirochete-like organism and the mycoplasma-like organism.